The development of the seminiferous tubule and initiation of the spermatogenic process are largely dictated by the interaction of FSH with the Sertoli cell. Most of the immature Sertoli cell functions are regulated by this gonadotropin. The response of this cell to FSH stimulation is, however, not constantly present throughout the life span of the Sertoli cell. It is modulated in parallel with testicular development and during the spermatogenic cycle. It has been established that FSH regulates cAMP intracellular levels by modulating phosphodiesterase activity. The aim of this research proposal is to elucidate the mechanisms involved in phosphodiesterase regulation. This will be accomplished by characterizing the two phosphodiesterase forms present in the soluble fraction of the Sertoli cell. The properties of the Ca++, calmodulin-dependent phosphodiesterase, whose activity is inhibited by FSH, will be studied using available polyclonal antibodies. The state of phosphorylation of this phosphodiesterase will be established, and the effect of gonadotropin on the phosphorylation and on the enzyme activity will be compared. It is further proposed to purify the enzyme and raise monoclonal antibodies against the high affinity cAMP phosphodiesterase, whose activity is stimulated by FSH. By these means, the structure of the enzyme will be characterized and the effect of FSH on the turnover of this enzyme will be determined. Isolation of cDNA clones coding for the phosphodiesterase will allow us to gain further information about the structure of the enzyme and to study regulation of mRNA levels. In order to assess the physiological importance of these FSH-dependent regulations, the role of phosphodiesterase in the changes in the Sertoli cell response occurring during testicular development and during the spermatogenic cycle will be determined. These studies will improve the understanding of the mechanisms of gonadotropin action and will offer new insight into events involved in the control of mammalian spermatogenesis.